State the law of conservation of energy.

$A$ body is moving with constant speed over a frictionless surface. What is the work done by the weight of the body?

Is it possible that a body be in accelerated motion under a force acting on the body,yet no work is being done by the force? Explain your answer giving a suitable example.

List two conditions which need to be satisfied for the work to be done on an object.

$(a)$ Define power. Give its $SI$ unit.
$(b)$ Taking the example of a simple pendulum, explain the variations in the forms of energy and the interconversions involved.

$A$ mass of $10 \, kg$ is dropped from a height of $50 \, cm$. Find its:
$(a)$ Potential energy just before dropping.
$(b)$ Kinetic energy just on touching the ground.
$(c)$ Velocity with which it hits the ground. (Given: $g = 10 \, m s^{-2}$)